1. Field of the Invention
The present invention is addressed to the production of metal and more particularly to the production of pure metal and metallic alloys being essentially free from slag or inclusions.
2. Discussion of Background
Non-metallic particles which are, for example, slag and which are known throughout the industry as "inclusions" within metal result from oxidation of active metallic elements during the molten metal refining and transport process and from the mechanical abrasion of refractories which are used to contain the molten metal. These inclusions may also be produced by being carried out from the starting change materials' such as ore or scrap or master alloys of ore or scrap which is used to make the metal or the alloy.
The removal of these inclusions or particles is essential to appearance and many times affects the mechanical properties of products made from the metal or the alloy. The recent significant increase in the demand for inclusion free metal has led to an increase in the effort to remove an ever decreasing size of inclusion from the metal.
The most common prior art techniques involved tundish and ladle refining by means of slag formation and subsequent removal as well as filtration of the particles from the molten metal system during transport to the mold. The ladle and tundish removal systems were commonly used for steel while filters were used for most other materials and the foundary industry.
The conventional filter systems involved removal by the action of refractory filters which were used to trap the inclusions. These filters worked on a combination of mechanical entrapment of the large particles and surface attraction of small inclusions which resulted from surface chemistry reactions. In any of these filter operations, as the filters were being used, their surfaces would begin to build up with the filter cake and eventually clogging occurred. As more and more clogging occurred, the flow rates of the molten metal through the filter progressively decreased until it was necessary to remove the filters and replace them with clean filters.
One of the other additional factors in the prior art filtering system is that these filters have difficulty removing very small particles (less than 40 microns) from the metal. This is true because the large particles or inclusions (greater than 50 microns) would clog the filter and thus make it ineffective in the removal of the smaller inclusions.
With this in mind, recent efforts have been addressed to a method of removing the large inclusions upstream of the filter in the molten flow. One of the significant areas which has evolved, in the pursuit of the large inclusion removal prior to the filter, is the use of electromagnetic fields. The first use or rather the concept of using the electromagnetic fields for the purification of molten metals was originated by Verte, Author's Certificate No. 141592-13c, Byull Isobert 19 (1961). The basic principle behind this process of Verte is similar to that of electromagnetic benefication of minerals as disclosed for example by U. Andres, Magonetohydrodynamic and Magonetohydrostatic Methods of Mineral Separation. John Wiley and Sons, 1976. The system of Verte, like the systems for Electromagnetic Separation of Minerals, produces a pressure gradient in the melt from an irrotational electromagnetic force field in order to accelerate the flotation of the inclusion particles.
Another procedure is shown in the system of R. Moreau, et al. Brevet Francais No. 79.08503, (1979). This system involves the developing of a separation process in which the electromagnetic force field in the system is generated by crossing the electric field produced by a passage of a D.C. current between two immersed electrodes in the melt with a uniform magnetic field. Although the feasibility of the process has been proven in the laboratory as detailed in the Proceedings of the Symposium on "Metallurgical Applications of Magnetohydrodynamics," by Marty and Alemany, The Metals Society, (1982), page 245, there still remains many technological problems when the process is scaled up to a system which would be necessary for commercial use. Most of these problems have to do with the electric field which must be produced in the melt and the effect on the melt.
One of the more recent developments in this area has been produced at Oak Ridge National Laboratory where there has been developed an electromagnetic separation process in which the electromagnetic forces in the melt are generated by induction from a time varying magnetic field. Although such technique eliminates the problem of melt contamination from the electrodes and although such technique is easy to implement, there remains significant problems because there is a lack of knowledge concerning the coupling between the electric and the magnetic field and the homogeniety of the electromagnetic force field. This force field is very important to the separation process because the vorticity of a force field would have a significant detrimental effect on the separation efficiency of the system. This is true because the force field vorticity will stir the melt and cause entrapment of the inclusion particles within the flow eddies. Thus, although Oak Ridge (ORNL) has produced a device which is capable of removing large particles (greater than 250 microns according to their results), significant problems still exist. The results of the ORNL device have been reported in an article by N. ElKaddah entitled "A Comprehensive Mathematical Model of Electromagnetic Separation of Inclusions in Molten Metals", I.E.E.E. Industry Application Society Annual Meeting, Pittsburgh, Oct. 2-7, Conference Records p. 1161, 1988. One of the immediately obvious problems with this system is that it removes inclusions only greater than 250 microns and furthermore, a significant shortcoming exists in that the vorticity of the electromagnetic force field established vigorously stirs the melt, which causes entrapment of inclusion particles within the flow eddies, as discussed above and carries them into the flowing stream, instead of separating them from the stream.
These large particles which become entrapped in the flow eddies created by the electromagnetic force have plagued all improvements in this area as well as in the area of separation of minerals when electromagnetic fields are used.